Wireless communications systems are used in a variety of telecommunications systems, television, radio and other media systems, data communication networks, and other systems to convey information between remote points using wireless transmitters and wireless receivers. A transmitter is an electronic device which, usually with the aid of an antenna, propagates an electromagnetic signal such as radio, television, or other telecommunications. Transmitters often include signal amplifiers which receive a radio-frequency or other signal, amplify the signal by a predetermined gain, and communicate the amplified signal. On the other hand, a receiver is an electronic device which, also usually with the aid of an antenna, receives and processes a wireless electromagnetic signal. In certain instances, a transmitter and receiver may be combined into a single device called a transceiver.
3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) and LTE-Advanced standards are default standards for 4th generation wireless communication networks. LTE and LTE-Advanced utilize single carrier orthogonal frequency-division multiple access (SC-FDMA) as the basis for the uplink PHY layer of a wireless communications device. SC-FDMA is a particular type of orthogonal frequency-division multiplexing (OFDM).
An wireless communication network may be configured to combine modulated signals at various frequencies (also known as “channels”). Each disparate channel may include encoded information to be communicated throughout the wireless communication network. Each frequency (or carrier) may carry multiple sub-carriers using frequency-division multiplexing (FDM).
Orthogonal frequency-division multiplexing (OFDM) is a FDM scheme in which a plurality of closely-spaced orthogonal sub-carriers is used to carry data. The data is divided into several parallel data channels, one for each sub-carrier. OFDM modulation may be implemented using inverse discrete Fourier transformation (IDFT) and a modulator instead of using multiple modulators and oscillators for subcarriers as is the case in traditional FDM. The demodulation is also achieved using discrete Fourier transformation (DFT) instead of using multiple filters and oscillators for subcarriers. The separation of subcarrier channels is the integer multiple of the inverse observation period for a symbol to assure orthogonality.
A conventional SC-FDMA transmitter allocates its transmission power evenly in all of the subcarriers it uses. Because different subcarriers may experience varying levels of noise and interference, the signal-to-interference and noise (SINR) ratio may vary across subcarriers, and certain subcarriers may have relatively low SINR.